The neurogenetic diseases typically involve deficiencies that affect cells throughout the brain. As a result, pathological lesions are widely distributed and treatment requires correspondingly widespread correction. The long-term goal of our studies is to develop a gene therapy strategy that is life-long and can arrest or reverse pathology in a meaningful volume of brain tissue. A number of potentially promising methods for delivering a therapeutic gene to the brain are under development, both from peripheral sites and direct transfer into the CNS. In this project we will focus on exploiting the natural network of neuronal pathways within the CNS to transport a gene and therapeutic protein to distal sites. The goal is to maximize the distribution of the functional protein while minimizing the number of injection sites. This will be important for translating the animal experimental findings into clinical application that minimizes risk but at the same time achieves sufficient distribution of the therapeutic gene to be medically effective in a human-size brain. This hypothesis is based on the results generated by this grant, which have shown that widespread correction of the metabolic defect can be achieved, in principle, by exploiting axonal transport. The protein can be transported over wide distances but only in some neuronal pathways. Furthermore, AAV vectors can be transported to distal sites, but only by certain serotypes and not in all pathways. In the current grant period we showed that targeting a pathway with wide projections and choosing a specific serotype of AAV can greatly expand the amount of brain tissue that is corrected after a single small injection in the mouse. As a test system for scaling up treatments 100-fold in brain size, we will use a cat model with the same lysosomal disease as the mouse. This coherent model system was chosen because the enzymatically active normal protein can be detected in situ with a very sensitive histochemical reaction, which has proven to be exceptionally useful for following the fate of gene transfer and enzyme distribution in three dimensions in the brain. Many of the principles our lab has demonstrated first in this model system have been used subsequently to study other lysosomal diseases as well as other neurogenetic disorders. While progress has been made, we need better understanding of the properties of AAV gene transport in neurons. Thus, we will: 1) study the cell biology of AAV trafficking in neurons in vitro using microfluidic cultures; 2) study additional pathways for AAV transport in the mouse and evaluate the utility of a limited number of the best candidate pathways in the cat brain; 3) evaluate a chimeric protein containing the TTC moiety that can transport the protein greater distances than the wild type enzyme; and 4) determine the effect of the treatments on pathology at the cell and molecular as well as histological levels. The principles of gene and protein transport being investigated in this project will potentially be useful for many genetic as well as other types of disseminated lesions in brain diseases.